The present description and claims use the expression "biological treatment zone" for an area or a section of for instance a container, a vessel, a reactor, a channel or a tank, wherein biologically active sludge is kept suspended by way of for instance a mechanical stirring or aeration under such aerobic or anaerobic conditions that the microorganisms present in the sludge consume the impurities in the water.
By the expression "sedimentation zone" is meant an area or section of for instance a container, a vessel, a reactor, a channel or a tank which is only exposed to an insignificant or no mechanical effect at all in such a manner that active sludge can settle.
By the expression "clarifying zone" is meant an area or section of for instance a container, a vessel, a reactor, a channel or a tank in which active sludge applies essentially in the settled state, and from which the cleaned water is decanted and drained off.
The expression "back-mix" is used for any process allowing a transfer of material from a first zone to a second zone while material is simultaneously being transferred from the second zone into the first zone. The "back-mix ratio" of a predetermined clarifying or sedimentation zone during a specific period is defined as the ratio of the volume amount of material (water with sludge suspended therein) being transferred with back-mixing from said zone during said period to another zone relative to the total volume of said clarifying and sedimentation zone.
DK-PS No. 131,181 discloses a method of the above type, where said method is ordinarily called the triple channel method or the T-channel method. This method employs three containers 1, 2 and 3. One container (container 2) operates as a permanent aeration zone, and the two remaining containers (i.e. containers 1 and 3) operate variably as aeration zone and clarifying zone. The three containers are connected to a common source for the feeding of waste water in such a manner that the feeding of waste water can alternate between the three containers. The containers are furthermore interconnected two by two.
When the triple channel is operated, container 1 operates during a first period as an aeration zone being fed with waste water while container 3 operates as a clarifying zone from which cleaned water is drained out. During a succeeding second period, waste water is fed to container 2 while container 1 still operates as an aeration zone and container 3 as a clarifying zone from which cleaned water is drained out. During a third period, the aeration is stopped in container 1 while waste water is still being fed to container 2 and cleaned water is being drained out of container 3. Subsequently, during a fourth period waste water is fed to container 3 which during this period operates as an aeration zone while container 1 operates as a clarifying zone from which cleaned water is drained out. During a fifth period, the waste water is fed to container 2 while container 3 still operates as an aeration zone and container 1 as a clarifying zone from which cleaned water is drained out. Finally during a sixth period, the aeration in container 3 is stopped while waste water is still being fed to container 2 and cleaned water is drained out of container 1.
This cycle including 6 periods is repeated over and over again. The patent description refers to a preferred embodiment where the total operational cycle lasts 8 hours. The periods 1 and 3 lasts 2 times 2 hours, and the periods 2, 3, 5, and 6 last 4 times 1 hour.
DK-PS No. 131,279 describes a method for denitrification of waste water by employing a triple channel system. This method is usually called the "BioDenitro"-method and is based on the same operational pattern as the triple channel described above. The biological treatment is, however, carried out in container 1 and 3 and alternately under anaerobic and aerobic conditions. Aerobic conditions are provided by the container in question being subjected to an aeration, whereas anaerobic conditions are provided by a relatively gentle stirring being performed in the container. Both functions can according to this Patent Specification be provided by a rotor installed in the container, said rotor running at a variable speed.
When a container is run under aerobic conditions, the NH.sub.3 --N present in the waste water is nitrified into NO.sub.3 --N. When the same container is run anaerobically during a later operational period, a denitrification takes place, i.e. the present NO.sub.3 --N is converted into free N.sub.2 escaping into the atmosphere.
Owing to the continuously increasing requirements presented to denitrification of waste water, the T-channel method involving the BioDenitro-running is extensively used.
The T-channel method is, however, encumbered with a number of draw-backs:
During some operational periods, the waste water is fed to a container which later on is used as a clarifying container. As the draining out is carried out from this container, the aeration must continue for a period after termination of the feeding of waste water to said container in order to ensure that the required cleanness of the outlet is obtained.
The utilization of the aerators is unfortunately relatively low. When polluted water is fed to the clarifying zones, the oxygen demand applying to the indicated operational pattern is distributed with 31% to each of the clarifying zones and 38% to the permanent biological treatment zone. The degree of utilization of the total aeration installation is therefore only 49%, because 62% of the oxygen demand applies to the zones where the aerators are stopped part of the time, i.e. 5 out of 8 hours in the operational pattern dealt with.
Furthermore, the mean concentration of sludge (i.e. the average of the three containers) possible at the T-channel method is relatively low because sludge has a tendency to accumulate in the side channels. During the four hours where sludge is fed to the side channel (container 1 or 3), so much sludge can thus accumulate that the critical concentration of sludge of approximately 7 kg/m is easily exceeded when an mean concentration of sludge of more than approximately 4 kg/m.sup.3 is involved.
DK-PS No. 123,814 discloses a method of cleaning waste water, said method ensuring a much better utilization of the aerators than the T-channel method. This method is ordinarily called the alternate channel or the V-channel method, and here a single container is divided into a main aeration part and two channel parts individually serving either as an aeration zone or as a clarifying zone. The aerators are arranged in the main aeration part continuously being fed with waste water. A typical operational pattern of this method includes the following four periods:
During a first three hour-period, one channel part operates as an aeration zone and the other channel part as a clarifying zone. Then the aerators are stopped for one hour during a second period. During a third three hour-period, the second channel part operates as an aeration zone and the first channel part as a clarifying zone. Then the aerators are stopped for one hour during a fourth period, and the described operational cycle is repeated.
In the V-channel method, the aerators are only stopped during the periods involving preparation of one channel part for the clarifying function, and the described operational pattern results in a degree of utilization of the aerators of 75%.
Despite the good degree of utilization of the aerators, the V-channel method is only used in a few waste water cleaning plants mainly on account of the structural limitations. In order to establish the best hydraulic conditions allowing the central aerators to circulate in two zones, the degree of freedom applying to the structural embodiment is limited, and it is not a good idea to position more than maximum two aerators in a plant section.
Another draw-back of the V-channel method is that the building depth is limited to 2.5 to 3 m, where large plants have a demand for building depths of 4 to 8 m in order to reduce the initial expenditure and the area need. Furthermore, like the T-channel method an extensive accumulation of sludge takes place during the relatively long clarifying period inside the clarifying zone with the effect that the rating of the plant for a maximum mean concentration of sludge is subject to limitations. Both V and T-channels are ordinarily rated for a maximum mean concentration of sludge of approximately 4 kg/m.sup.3.
Another known method ensuring a high utilization of the aerators appears from FIG. 1 of the drawing. In this method, waste water is fed into a biological reactor including aeration units. The clarification is carried out by a permanent post-clarifying vessel, and the aerators in the biological reactor can therefore be utilized 100% of the time. The sludge being separated in the post-clarifying vessel must be returned by way of pumping to the biological reactor. The latter return flow increases, however, the water flow from the biological reactor to the post-clarifying vessel and consequently the amount of sludge transported thereto and separated therein is increased as well. The sedimentation inside the post-clarifying vessel results in a concentration of the sludge. This process is, however, carried out relatively slowly, and in practice it is not possible to achieve a concentration of sludge in the return flow of more than 8 to 10 kg/m.sup.3.
The following Table I illustrates the necessary returning relative to the feeding of waste water versus the concentration of sludge in the biological reactor of FIG. 1 and under the assumption of a concentration of sludge in the return flow of 8 kg/m.sup.3.
TABLE I Concentration of sludge in biological reactor Return flow relative to (kg/m.sup.3) feeding of waste water 3.5 0.75 4.0 1.0 4.5 1.25 5.0 1.5 5.5 2.1
Usually a plant is rated to carry out the described method of maintaining a concentration of sludge of 3.5 to 4 kg/m.sup.3 in the biological reactor. As illustrated in Table I, this situation requires, however, a return flow of approximately the same volume as the feeding of waste water, which, of course, subjects the capacity of such a plant to an extensive limitation. Furthermore, it is necessary to include a return pump station of a high capacity.
DK-PS No. 131,279 describes a plant comprising two biological reactors which alternately are run aerobically and anaerobically, as well as a permanent post-clarifying vessel. This plant allows a denitrification of waste water, but it is, however, encumbered with the same draw-backs as the plant described above as far as the capacity is concerned. Moreover, the denitrification is carried out in only one step. However, when a very low concentration of impurities is desired it turned out to be very advantageous to carry out the denitrification in two steps, where the first step operates at high rates and a relatively high concentration of impurities.